About a decade ago, for the first time in history, the number of people living in urban areas surpassed that of those living in rural ones. But “urban” does not mean New York or Beijing or Rome. About half the urban population still lives in fairly small cities of fewer than 500,000 people (at least in developing countries) that may resemble rural areas more than mega-cities. Europe, for instance, has just two mega-cities and many smaller cities.

There are already 29 mega-cities with populations of 10 million or more—including Delhi, Shanghai, Mexico City, Sao Paulo, Lagos and Kinshasa—but they make up just 12% of the global urban population. By 2035, we’re expected to have 50 mega-cities, but they would only account for 16% of all urban dwellers.

What’s more, urbanization has not advanced at the same pace in all regions. Europe and Northern America urbanized early, and their populations are already mostly urban (74.5% and 82% respectively). So are those of Latin America and the Caribbean, 81% of whose inhabitants live in urban areas. In sharp contrast, Africa’s population is still mostly rural (57%) and Asia’s has just become 50% urban.

Asia’s urbanization levels are largely determined by those of the two population giants, China and India. Until 1990, they were among the least urbanized countries in the world, with only 25% of their respective populations living in cities. Since then, China’s economic transformation has been accompanied by very rapid urbanization: China is expected to be three-quarters urban by 2038, up from 60% today. India, by contrast, still lags far behind with just about a third of its population living in cities, a proportion expected to rise to 45% by 2038.

High urbanization levels are associated with higher GDP. As the experience of China shows, rapid economic growth tends to accelerate urbanization. When high shares of the population make their living from agriculture, the productivity of that sector tends to be low. By contrast, during economic development, the most dynamic sectors of the economy tend to cluster in urban centers—or even give rise to them.

In China, for instance, the economic liberalization that began in 1978 promoted the development of enterprises in rural villages, which led to an economic boom in rural areas. The growth of rural enterprises spurred the development of new towns and cities by making villages become increasingly urban. As a consequence, the number of cities in China grew from 193 in 1978 to 655 in 2008, with the majority of new cities being small or medium-sized. The emergence of so many new cities—many located near the rural areas from which they derived their dynamism—helped reduce the impact of rural-to-urban migration on the large cities of China.

The movement of people from rural to urban areas is only one of the ways in which urban populations grow. Additions to the urban population also happen because births exceed deaths in urban areas, or because new cities emerge or existing cities expand, often encompassing former rural settlements. In some of the least developed countries, urban populations increase mainly because urban couples have many children who survive to be adults.

Rural-to-urban migration has not in general been the major contributor to urban population growth in developing countries.

For instance, in Niger, where the population is mostly rural (84%), the number of urban dwellers is doubling every 17 years because fertility is still a high seven children per women. Similarly, in much of Africa, high fertility is fueling rapid urban population growth, implying that increasing urbanization in the region is often not indicative of economic dynamism.

Demographers estimate that in most developing countries since the 1960s, the excess of births over deaths has accounted for well over half of the population increase in urban areas. Therefore, rural-to-urban migration, though significant over certain periods, has not in general been the major contributor to urban population growth in developing countries. Furthermore, in highly urbanized countries the majority of internal migrants already originate in cities and simply move to other cities, therefore having no impact on the overall size of the urban population. That is the case in the United States, in most European countries, and in highly urbanized developing countries, such as Brazil.

Urbanization is mostly positive. Evidence from developing countries shows that, on average, people living in urban areas are better off than rural dwellers. Because urbanites have better access to health care, they have better health and live longer than rural dwellers; their educational attainment is higher because educational institutions are better and more easily accessible in urban than in rural areas; and they benefit from a more diversified labor market than that typical of rural areas. Nevertheless, cities in developing countries are not free from stresses: high levels of underemployment, the growth of slums, lack of adequate infrastructure, and costly services are problems that remain on the agenda of countless cities.

The expected expansion of cities in the developing world poses a number of challenges, including the necessity of generating decent jobs for their growing populations and providing them with adequate urban services in terms of housing, water and sanitation, transportation, electrification, nutrition, education, and health care. Furthermore, over the next few decades, cities will have to increase their resilience to the consequences of climate change, especially considering that many populous cities—such as Shanghai, Osaka, Mumbai, New York, Miami, Rio de Janeiro, Alexandria, and Durban—are located in coastal areas that are very likely to be affected by rising sea levels. Though a few of the coastal cities are beginning to take measures to increase their resilience to floods and storm surges, if the average global temperature increases beyond 2° celsius, large tracts of urban land will be submerged and people will have to move elsewhere.

Technology and economies of scale may facilitate addressing some of these challenges. But in most countries, proactive planning for ensuring the resilience of urban centers is still the exception rather than the rule. Innovative approaches will be necessary to ensure that urban centers may continue to offer the best chances of enjoying long and productive lives. These approaches will require educating and nudging people to practice resource conservation, especially with regard to energy and water use. Technology may provide some solutions but it is ultimately the adoption and consistent use of appropriate technologies by each of us that will make a difference.

(Note: All statistics cited in this piece are derived from World Urbanization Prospects: The 2018 Revision, produced by the Population Division of the United Nations.)